This invention relates generally to examining objects or zones. In one aspect, the invention relates to sensing (i.e. detecting) a selected narrow frequency band of radiation which can be received from any point along an extended line, using narrow spectral band filtering (narrow band pass filter means). The invention is more particularly but not exclusively for identifying specific discrete objects or specific zones of an article. The invention was developed for sorting gemstones, and specifically diamonds, from gemstone-bearing ore; it may be applicable to sorting other gemstones or minerals, such as emeralds, rubies or zircons. However, the invention can be used as a general technique for examining a large area, and less generally can be applied to identifying any suitable discrete objects, or can be applied to general inspection techniques such as inspecting paper sheet material or quality control of castings or turbine blades, or examining metals for impurities, e.g. slag in steel, or detecting a gap in an anti-reflection coating on glass or in a diamond film on a loudspeaker cone, or examining filleted fish for freshness or the presence of bones (using ultra-violet radiation).
Much of the remainder of the description is particularly concerned with sensing or detecting Raman radiation on excitation with visible laser radiation, but the invention is applicable to any suitable exciting radiations, such as X-ray, visible, infra-red or ultra-violet radiation, produced by any suitable means. The emission can be detected in any suitable direction relative to the incident radiation, e.g. in the same direction (back illumination) or in the opposite direction (front illumination).
It is known that when certain materials are irradiated, in addition to scattering the incident radiation, they emit radiation in the form of broad band fluorescence (wavelengths longer than the excitation wavelength), and in discrete frequencies different due to the Raman shift. The Raman frequency bands (called the Stokes and the anti-Stokes) are different from and equally spaced on either side of the frequency of the incident radiation; the frequency differences are uniquely characteristic of a material. These Raman emissions enable e.g. diamond to be identified and sorted from other materials such as spinel, calcite and zircons. Although there are two Raman frequencies, one normally looks at the lower frequency (longer wavelength) Stokes emission as it has the greater intensity under normal operating conditions.
Normally, the exciting radiation not only causes the diamond Raman emissions, but also excites other luminescences. The gangue does not exhibit Raman emission with a frequency shift characteristics of diamond. However gangue, and some diamonds, emit other wavelength radiation or fluorescence, and this gives considerable problems in identifying only the Raman radiation and hence the diamonds. The Raman emission is very weak, and can be completely swamped by the other emitted radiations.
The possibility of using the Raman shift to sort or identify diamonds has been described in general terms in for instance GB-A-2 140 555, GB-A-2 199 657, WO 86/07457 and WO 88/01378.
Another problem with using the Raman shift is that as the Raman emission is very weak, a large aperture lens or other collection means must be used to capture the maximum amount of Raman radiation--in general, one needs a lens of say f1 or less. A further problem is that if the method is to be used commercially, large numbers of objects must be sorted per unit time, or large areas of the articles must be scanned per unit time; for example, when sorting ore, one should be able to sort ore which is travelling on a belt at least 0.3 m wide and generally say 1 m or 2 m wide--the particles of ore can occupy a wide path in other ways, for instance if sliding, falling or in free flight or if carried in a liquid stream. Very generally, it is desirable to be able to sort particles or objects moving in a path whose width can accommodate a number of the particles or objects. WO 86/07457 does not deal with this problem, as it is concerned with the identification of a diamond by a jeweller. GB-A-2 140 555 and GB-A-2 199 657 describe ore sorting, but the machinery used requires the ore to be fed along a narrow belt so that the ore particles are lined up in the direction of travel, and each particle is passed through the optical axis of the viewing means. WO 88/01378 uses a multiplicity of optical paths to cover a wide conveyor chute, each path being confined and being its own detector.
Normally, the exciting radiation not only causes the Raman emissions, but also excites the general background radiation. The Raman radiation is also in a very narrow band, so it is possible to reduce general background radiation using a commercially-available narrow band pass filter having a narrow pass band. In this context, "narrow" has its normal meaning as used in this art. However, more specifically, it can mean selecting a band of wavelengths which, on an energy/wave length curve, extends approximately from half-amplitude on one side of the emission being examined to half amplitude on the other side. For the invention, and particularly for Raman, the band will normally be of the order of 1 nm, say 1 nm or 2 nm, and is most unlikely ever to be greater than 10 nm. For other photoluminescence, the band could be approximately 20, 30 or 40 nm. The filters used, particularly for narrow band filtering, will normally be interference filters where the band is transmitted; in theory at least, a reflected narrow band could be sensed. Narrow band pass filters are also called line filters.
A narrow band pass filter however passes its design pass frequency on its axis (zero angle of incidence), but passes slightly different frequencies off its axis. This is illustrated in FIG. 1 of the accompanying drawings. In other words, the pass frequency of the filter depends upon the angle of incidence and it is necessary for all rays to pass through the filter nearly parallel to the axis, one quoted maximum divergence being .+-.4.degree.--in practice, the specific angle depends upon how sensitive the detection should be, and wider or narrower divergences may be acceptable. If rays pass through the filter at greater angles, it is possible for non-diamond material to be identified as diamond material. This does not give a problem when the objects are on the optical axis, but it does give a considerable problem when the objects are distributed over a relatively wide area. More generally, there is a danger, when sorting objects or marking defective zones of an article, that the wrong object or zone is selected due to picking up an oblique emission from an object or zone, of the wrong but adjacent wave length.